Hsp90 is thought to "chaperone" steroid associates with v-src in the cytoplasm and is thought to be involved in the "maturation" of this and other classes of nonreceptor tyrosine kinases. In order to examine the mechanisms by which these ubiquitous proteins chaperone protein folding and regulate signal transduction, we have overexpressed, purified, and crystallized htpG (high temperature production protein G), the Escherichia coli hsp90 class protein.